This invention generally relates to interferometry and more particularly to interferometric apparatus and methods by which highly precise metrology is practiced.
In highly precise metrology, it is required to understand and compensate for changes in the direction of propagation of various individual light beams in which information is encoded about distances or angles under measurement. Changes in the direction of propagation of such beams or in their angular relationships with respect to one another or some reference are often introduced by pitch, yaw, and roll of various optical components in the interferometer as, for example, those that might be experienced by a quickly slewing planar measurement mirror in a plane mirror interferometer, or the like. If such effects are not compensated, it is often not possible to achieve the desired precision required of important industrial applications such as those practiced in the fabrication of semiconductor devices.
In addition, where interferometers have dynamic elements that may be used to control the direction of propagating beams, some means for measuring errors in beam direction and a measurement of a change in an angle is needed to provide feedback signals for controlling such elements.
Consequently, it is a primary object of the present invention to provide apparatus and methods for measuring and/or controlling the differential direction of propagation and/or angular attitude among one or more light beams.
It is another object of this invention to provide apparatus and methods by which changes in the direction of propagation and/or angular relationships among one or more light beams may be measured with interferometric precision.
It is another object of this invention to provide apparatus and methods by which changes in the angular orientation of an object may be measured with interferometric precision.
Other objects of the invention will in part appear hereinafter and will in part be obvious when reading the following detailed description in connection with the accompanying drawings.
The present invention relates to apparatus and method(s) for interferometric measurement of a change in the relative directions of propagation of components of an optical beam, for interferometric measurement of a change in the direction of propagation of an optical beam, and for interferometric measurements of the change in orientation of an object.
In one aspect, the invention comprises apparatus for measuring differences between the angular directions of travel of light beams and comprises a multiple beam interferometer, the multiple beam interferometer being arranged to intercept the light beams to produce an output beam; a lens for focusing the output beam to a spot in a detector plane; and a detector located in the detector plane for receiving the focused output beam and generating an electrical signal having a phase that varies in at least one plane in accordance with the angular separation between the light beams; and electronic means for receiving the electrical signal, determining the phase therefrom, and converting the phase to the angle separating the light beams.
In another aspect, optical alignment means are provided for assuring that at least one of said light beams enters the multiple beam interferometer aligned in a predetermined attitude with respect to the multiple beam interferometer such that the phase of said electrical signal is insensitive to changes in the angular direction of travel of the at least one of the light beams aligned in the predetermined attitude with respect to the multiple beam interferometer whereby the multiple beam interferometer serves as a reference from which an angular change of the other light beam may be measured.
Another aspect includes means for modulating an optical path length in the multiple beam interferometer so that the phase of the electrical signal varies with the frequency of modulation, the amplitude of the modulation, the optical path length in the multiple beam interferometer, and the angular separation between the light beams.
Another aspect includes means for providing a feedback signal derived from the electrical signal and for adjusting the optical path length of the multiple beam interferometer to optimize the magnitude of the electrical signal such that it is optimal at an optical path length where the optical path length of the multiple beam interferometer is an integer multiple of 2xcfx80 at the operating wavelengths of the light beams.
Another aspect includes a distance measuring interferometer for measuring the optical path length of the multiple beam interferometer as the optical path length of the multiple beam interferometer is adjusted.
Another aspect provides the electronic means with means for determining the relative angle between the light beams when the electrical signal has been optimized.
Another aspect involves various steps of method(s) for measuring differences between the angular directions of travel of light beams.